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Stock-based Compensatin (Stock Options) (Details) - CAD ($) $ / shares in Units, $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] |  |  |  |
Exercised |  | (20,000) | (100,000) |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period |  | 20,000 | 100,000 |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value |  | $ 1.7 | $ 2.37 |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options |  | $ 5 | $ 3 |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition |  | 2 years 2 months 13 days | 2 years 6 months |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term |  | 5 years 1 month 6 days | 5 years 4 months 25 days |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value |  | $ 18 | $ 22 |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value |  | $ 2 | $ 2 |
Employee Stock Option [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] |  |  |  |
Beginning Balance |  | 3,643,575 |  |
Granted |  | 627,600 |  |
Exercised |  | (23,800) |  |
Forfeited |  | (11,700) |  |
Expired |  | (10,100) |  |
Ending Balance |  | 4,225,575 | 3,643,575 |
Options exercisable | [1],[2] | 2,546,250 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] |  |  |  |
Beginning Balance |  | $ 39.42 |  |
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price |  | 39.93 |  |
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price |  | 24.98 |  |
ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice |  | 45.1 |  |
ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice |  | 39.93 |  |
Ending Balance |  | 39.56 | $ 39.42 |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price |  | $ 37.15 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross |  | 627,600 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period |  | 23,800 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] |  |  |  |
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price |  | $ 24.98 |  |
Employee Non-vested Stock Option [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] |  |  |  |
Granted |  | 627,600 |  |
Vested |  | (666,125) |  |
Forfeited |  | (11,700) |  |
Expired |  | (10,100) |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] |  |  |  |
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price |  | $ 1.7 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Weighted Average Grant Date Fair Value |  | 2.51 |  |
ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice |  | 2.54 |  |
ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice |  | $ 1.7 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |
Beginning Balance | [3] | 1,739,650 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross |  | 627,600 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Number of Shares |  | 666,125 |  |
Ending Balance |  | 1,679,325 | 1,739,650 | [3]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] |  |  |  |
Beginning Balance |  | $ 2.52 |  |
Sharebased Compensation Arrangement By Sharebased Payment Award Options Nonvested Options Forfeited Weighted Average Grant Date Fair Value |  | 2.54 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Weighted Average Grant Date Fair Value |  | 2.51 |  |
Ending Balance |  | $ 2.22 | $ 2.52 |
[1] | As at December 31, 2018, the fair value of options that vested in the year was $2 million (2017 - $2 million).
[2] | As at December 31, 2018, the weighted average remaining term of vested options was 5.1 years with an aggregate intrinsic value of $18 million (2017 - 5.4 years, $22 million).
[3] | As at December 31, 2018, there was $5 million of unrecognized compensation related to stock options not yet vested which is expected to be recognized over a weighted average period of approximately 2.2 years (2017 - $3 million, 2.5 years).

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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Unrecognized cost of unvested options awarded to employees as compensation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Number of options outstanding, including both vested and non-vested options. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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